The present invention generally relates a controlled frequency generator and, more specifically, to a multi-stage controlled frequency generator for direct-drive wind power.
Wind power is a fast growing renewable energy source. Various wind turbine concepts have been developed with the goal of maximizing the energy harnessed, while minimizing the installation and operation costs and providing utility grid equivalent electrical power quality. Such turbine concepts can be classified into geared and direct-drive systems.
FIG. 1 shows a functional block diagram of a conventional geared system 100. The major components of the geared system 100 include a wind turbine 102, a generator 104, a power converter 106, and a gearbox 108. Typically the generator 104 used in geared systems are of the doubly-fed (induction) machine type (DFM). The power converter 106 required to control this type of machine has to be rated at approximately 25% of the total generated power when the generator shaft speed varies by +/−33% around its synchronous speed.
FIG. 2 shows a functional block diagram of a conventional direct-drive system 200. The major components of the direct-drive system 200 include a wind turbine 202, a generator 204, and a power converter 206. Typically the generator 204 used in direct drive systems are synchronous generators (SG) either with wound rotor electrically excited or with permanent magnets. The power converter 206 required to connect these types of generators to the grid has to be rated at 100% of the total generated power.
In conventional wind power systems, referring back to FIG. 1, the gearbox 108 connects a low-speed shaft 110 moved by the blades of the turbine 102 to a high-speed shaft 112, thereby increasing the rotational speed from approximately 15-25 rotations per minute (rpm) to about 1000-1700 rpm at the generator shaft (high speed shaft 112). These values are typical for larger wind turbines over a megawatt (MW).
Over 20 years, the direct-drive generator 204 makes the same number of rotations that the generator 104 of the geared system 100 makes in about 16 weeks, resulting in significant reliability and operational life increase of the direct-drive generator system 200, before considering the additional moving parts in the gearbox 108. More significantly, the stress levels within the gearbox 108 of the geared system 100 may contribute to gearbox failure, especially when the effects of wind turbulence are considered.
Direct-drive generators constitute the state of the art of the industry because they eliminate the wind turbulence driven gearbox stresses. Because of the high torque and very low shaft speed (15 to 25 rpm) they are very large, heavy and expensive. The low speed operation of direct-drive wind power systems dictates a large number of rotor poles in the generator. This high number of rotor poles significantly increases the size, weight and cost of the generator. Literature shows that a 2 MW direct drive generator has a weight of about 48 tons.
Conventional direct-drive generators may also lack the ability to control the output frequency at 60 Hz. They typically have a low frequency of about 10 Hz. An electronic power controller (power converter) rated to convert all the power supplied by the generator is required to provide usable, utility quality power, that is connectable to the power grid. The large electronic controller adds significant cost and weight to the wind turbine system.
As can be seen, there is a need for a controlled frequency generator for direct-drive wind power.